Zinc secondary batteries have been developed and studied over many years. Unfortunately, these batteries have not yet been put into practice. This is due to a problem that zinc contained in the negative electrode forms dendritic crystals, i.e. dendrites, during a charge mode of the battery and the dendrites break the separator to cause short circuit between the negative electrode and the positive electrode. In contrast, nickel-cadmium batteries and nickel-hydrogen batteries have already been commercialized. Nickel-zinc secondary batteries, however, have advantages over such commercialized batteries. In specific, nickel-zinc secondary batteries have a very high theoretical density of capacity; i.e., about five times that of nickel-cadmium secondary batteries, 2.5 times that of nickel-hydrogen secondary batteries, and 1.3 times that of lithium ion batteries. In addition, nickel-zinc secondary batteries are produced from inexpensive raw materials. Thus, a strong demand has arisen for a technique for preventing the short circuit caused by dendritic zinc in zinc secondary batteries.
For example, Patent Document 1 (WO2013/118561) discloses a nickel-zinc secondary battery including a separator composed of a hydroxide-ion-conductive inorganic solid electrolyte between a positive electrode and a negative electrode for preventing the short circuit caused by dendritic zinc, wherein the inorganic solid electrolyte is a layered double hydroxide (LDH) having a basic composition represented by the formula: M2+1−xM3+x(OH)2An−x/n.mH2O (wherein M2+ represents at least one type of divalent cation, M3+ represents at least one type of trivalent cation, An− represents an n-valent anion, n is an integer of 1 or more, and x is 0.1 to 0.4).
Sealed nickel-zinc batteries have been disclosed which are provided with negative electrodes that absorb to recycle oxygen gas generated at the end of a charge mode. For example, Patent Document 2 (JPH05-303978A) discloses a sealed nickel-zinc battery including an electrode assembly including a positive electrode plate, a negative electrode plate, a separator, and a retainer, and a liquid-retainable layer disposed around the assembly, wherein the liquid-retainable layer is composed of a fibrous cellulose material having a length of 0.5 to 50 mm and a diameter of 5 to 100 μm and impregnated with an electrolytic solution. The separator used in the battery disclosed in Patent Document 2 is composed of a porous polypropylene membrane treated with a surfactant. Patent Document 3 (JPH06-96795A) discloses a sealed nickel-zinc battery including an electrode assembly, a battery container, and an electrolytic solution, wherein the negative electrode of the assembly faces the bottom of the container, and the electrolytic solution has a volume that is more than 98% and 110% or less of the total spatial volume of the electrode assembly. The separator used in the battery is composed of a microporous film and a cellophane membrane.
A technique has been disclosed for facilitating the permeation of oxygen gas generated from a positive electrode through a separator to a negative electrode during an overcharge mode of a battery. For example, Patent Document 4 (JPH05-36394A) discloses a separator for an alkaline battery, the separator being composed of a porous hydrophobic resin membrane having a surface coated with at least a hydrophilic fabric.